1. Field of the Invention
This invention is generally directed to improving the quality of bass sounds generated by one or more loudspeakers within a listening area. More particularly, the invention is directed to substantially equalizing the responses generated by at least one loudspeaker within a listening area so that the responses in the area are substantially constant and flat within a desired frequency range.
2. Related Art
Sound systems typically include loudspeakers that transform electrical signals into acoustic signals. The loudspeakers may include one or more transducers that produce a range of acoustic signals, such as high, mid and low-frequency signals. One type of loudspeaker is a subwoofer that may include a low frequency transducer to produce low-frequency signals in the range of 20 Hz to 100 Hz.
The sound systems may generate the acoustic signals in a variety of listening environments. Examples of listening environments include, but are not limited to, home listening rooms, home theaters, movie theaters, concert halls, vehicle interiors, recording studios, and the like. Typically, a listening environment includes single or multiple listening positions for a person or persons to hear the acoustic signals generated by the loudspeakers. The listening position may be a seated position, such as a section of a couch in a home theater environment, or a standing position, such as a spot where a conductor may stand in a concert hall.
The listening environment may affect the acoustic signals, including the low, mid, and/or high frequency signals at the listening positions. Depending on the nature of the room and the position of a listener in a room and the position of the loudspeaker in the room, the loudness of the sound can vary for different frequencies. This may especially be true for low frequencies. Low frequencies may be important to the enjoyment of music, movies, and most other forms of audio entertainment. In the home theater example, the room boundaries, including the walls, draperies, furniture, furnishings, and the like may affect the acoustic signals as they travel from the loudspeakers to the listening positions.
The acoustic signals received at the listening positions may be measured. One method of characterizing the room is the impulse response of a loudspeaker to a microphone placed in the listening area. The impulse response is the acoustic signal measured by the microphone for a short sound burst emitted from the loudspeaker. The impulse response may allow measurement of various properties of the acoustical signals including the amplitude and/or phase at a single frequency, a discrete number of frequencies, or a range of frequencies.
An amplitude response is a measurement of the loudness at the frequencies of interest. Generally, the loudness or the amplitude is measured in decibels (dB). Amplitude deviations may be expressed as positive or negative decibel values in relation to a designated target value. The closer the amplitude values measured at a listening position are to the target values, the better the amplitude response is. Deviations from the target reflect changes that occur in the acoustic signal as it interacts with room boundaries. Peaks represent a positive amplitude deviation from the target, while dips represent a negative amplitude deviation from the target.
These deviations in the amplitude response may depend on the frequency of the acoustic signal reproduced at the subwoofer, the subwoofer location, and the listener position. A listener may not hear low frequencies as they were recorded on the recording medium, such as a soundtrack or movie, but instead as they were distorted by the room boundaries. Thus, the room can change the acoustic signal that was reproduced by the subwoofer and adversely affect the low-frequency performance of the sound system. As an example, FIG. 1 shows a sound system setup in a rectangular room. The sound system includes a receiver connected to four subwoofers, one at each corner of the room. The room is defined by four walls that can affect the low-frequency sound waves or bass sounds generated by the four subwoofers. Within the room, a seating area is provided to allow one or more persons to listen to the combined bass sound generated by each of the four subwoofers. A number of factors, as discussed above, can affect the quality of the sound within the listening area such that one person may hear a louder bass sound than another person sitting just a few feet away. For purposes of measuring the impulse response of the room, the receiver may send a logarithmic frequency sweep output signals to the four subwoofers for a predetermined amount of time. The impulse responses of the room are then picked up by four microphones P1, P2, P3, and P4 positioned at different locations within the listening area of the room.
FIG. 2 shows four frequency response curves F1, F2, F3, and F4, corresponding to the measured impulse responses one may expect at the four microphone positions P1, P2, P3, and P4, respectively. As discussed earlier, subwoofers generally operate in the low frequency range of between 20 Hz and 100 Hz. FIG. 2 indicates that at about 48 Hz, the magnitude or loudness of the bass sound varies in a wide range so that the loudness of the bass sound depends on where the person is located within the listening area. For instance, the curve F2 indicates that the bass loudness levels is about 0 dB at about 48 Hz, while the curve F3 indicates that the bass loudness level is about −18 dB, at the same frequency point. This means that a person sitting in location P2 hears a much louder bass sound at 48 Hz than the person sitting just behind him at location P3. In other words, the sound level is not the same at different locations within the listening area of the room so that each person will experience a different bass sound quality. In addition, FIG. 2 shows that the curves fluctuate within the frequency range of interest. This means that certain bass sounds will drop off such that a person cannot hear the bass sound although it was intended to be heard. For instance, the curve F4 shows that between about 48 Hz and 55 Hz, there is a considerable drop in the bass loudness level at about 52 Hz. This means that a person sitting at location P3 will hear the bass sound at 48 Hz but notice a sudden drop in the bass sound at 52 Hz and a sudden peak again at 55 Hz. Such fluctuations in the bass sound level can impair the listening experience.
Many equalization techniques have been used in the past to reduce or remove amplitude deviations within a listening area. One of the techniques is spatial averaging that calculates an average amplitude response for multiple listening positions, and then equally implements the equalization for all subwoofers in the system. Spatial averaging, however, only corrects for a single “average listening position” that does not exist in reality. Thus, even when using spatial averaging techniques, some listening positions still have a better low-frequency performance than other positions but other locations may be severely affected. For instance, the spatial averaging may worsen the performance at some listening positions as compared to their un-equalized performance. Moreover, attempting to equalize and flatten the amplitude response for a single location potentially creates problems. While peaks may be reduced at the average listening position, attempting to amplify frequencies where dips occur requires significant additional acoustic output from the subwoofer, thus reducing the maximum acoustic output of the system and potentially creating large peaks in other areas of the room.
Another known equalization technique is to position multiple subwoofers in a “mode canceling” arrangement. By locating multiple loudspeakers symmetrically within the listening room, standing waves may be reduced by exploiting destructive and constructive interference. However, the symmetric “mode canceling” configuration assumes an idealized room (i.e., dimensionally and acoustically symmetric) and does not account for actual room characteristics including variations in shape or furnishings. Moreover, the symmetric positioning of the loudspeakers may not be a realistic or desirable configuration for the particular room setting.
Still another equalization technique is to configure the audio system in order to reduce amplitude deviations using mathematical analysis. One such mathematical analysis simulates standing waves in a room based on the room data. For example, room dimensions, such as length, width, and height of a room, are input and the various algorithms predict where to locate a subwoofer based on data input. However, this mathematical method does not account for the acoustical properties of a room's furniture, furnishings, composition, etc. For example, an interior wall having a masonry exterior may behave very differently in an acoustic sense than a wood framed wall. Further, this mathematical method cannot effectively compensate for partially enclosed rooms and may become computationally onerous if the room is not rectangular.
There are a number of other methods that try to equalize the impulse responses in a room but the accuracy of the equalization is more by chance because of the guessing involved in determining certain parameters such as delay and gain applied to the signals. As such, in order to obtain an accurate equalization solution, it takes a tremendous amount of computational power. Moreover, these methods do not provide an equalization that results in a flat frequency response within a desired low-frequency range so that loudness of the bass level is not only consistent at each seating location but also substantially constant or flat throughout the desired low-frequency range. Therefore, a long-standing need exists for a system to accurately determine a configuration for an audio system such that the audio performance for one or more listening positions in a given space is improved.